[338] LeetCode. Count bits (bit operation)

Given a non-negative integer num. For 0 ≤ i ≤ i, each digital num range, which calculates the number of binary digits 1 and returns them as an array.

Example 1:
Input: 2
Output: [0,1,1]

Example 2:
Input: 5
Output: [0,1,1,2,1,2]

Solution 1: use the library functions

class Solution {
    public int[] countBits(int num) {
        int[] arr = new int[num + 1];
        for(int i = 0; i <= num; i++){
            arr[i] = Integer.bitCount(i);
        }
        return arr;
    }
}

Solution two:

class Solution {
    public int[] countBits(int num) {
        int[] arr = new int[num + 1];
        for(int i = 1; i <= num; i++){
            arr[i] = arr[(i & i- 1)] + 1;
        }
        return arr;
    }
}